1. Field of the Invention
This invention relates generally to a multifunction printed antenna and, more particularly, to a multifunction printed antenna for wireless and telematic applications, including GPS, satellite radio, AMPS, PCS, GSM, etc., where multiple antenna elements are printed on a common circuit board.
2. Discussion of the Related Art
There is a growing demand for wireless communications services, such as cellular telephone, personal communications systems (PCS), global positioning systems (GPS), satellite radio, etc. With this demand comes a need for low-cost miniaturized planar antennas. The multitude of wireless services requires multiple antennas to cover the different frequency bands and functions. Also, the demand for dual-band phones is ever growing, as people increasingly tend to use both analog and digital communications services. Further, both cellular phone and PCS antennas require an omni-directional pattern.
Additionally, it is desirable that the size of the communication apparatus and the transmitting or receiving antennas be small. This becomes even more of a necessity when multiple antennas have to be mounted in a limited area. In military applications, a small antenna size is critical for low radar visibility, and to increase system survivability. In commercial applications, small size alleviates problems with styling, vandalism and aerodynamic performance. Size reduction is especially useful in low frequency applications in the HF, VHF, UHF and L frequency bands ranging from 30 to 3000 MHz. The wavelengths in these bands range from 10 m to 10 cm. Considering the fact that a resonant dipole is about a half-wavelength long, the motivation behind size reduction is obvious.
For low frequency applications, low-profile printed antennas include printed microstrip dipole and printed slot antennas. Printed antennas essentially comprise a printed circuit board with a trace layout. The trace layouts can be made using chemical etching, milling or other known methods. These antennas enjoy a host of advantages including ease of manufacture, low cost, low profile, conformality, etc.
FIGS. 1(a) and 1(b) show a known printed slot antenna 10 including a metallized ground plane 16 and a microstrip feed line 12 printed on opposite sides of a printed circuit board (PCB) 14. A linear slot element 18 is cut out of the ground plane 16 by a suitable etching step or the like. The microstrip line 12 is connected to the ground plane 16 at the edge of the slot element 18 by a shorting pin 20 extending through the PCB 14.
Various techniques are known in the art to reduce the size of a printed slot antenna of the type shown in FIGS. 1(a) and 1(b). For example, it is known to use dielectric lenses to reduce the size of a printed antenna. U.S. Pat. No. 6,081,239 issued Jun. 27, 2000 to Sabet et al. discloses a planar printed antenna that employs a high dielectric superstrate lens having a plurality of air voids that set the effective dielectric constant of the material of the lens to reduce resonant waves in the lens, thus reducing power loss in the antenna. The superstrate with air voids allows the size of the dipoles or slots to be reduced for any particular frequency band.
It is also possible to reduce the area occupied by a linear antenna element by bending or winding the antenna element into a curved or twisted shape. FIGS. 2(a) and 2(b) show a linear slot element 22 being wound to illustrate this technique. However, bending the antenna element 22 immediately results in a sharp reduction of its bandwidth. This can be verified by numerical modeling and computer simulation.
FIG. 3 shows the effect of gradually bending a slot antenna element 24 and how it affects the antenna bandwidth, near field, and vertical and horizontal polarization. This simulation shows that more windings result in a more omni-directional antenna pattern, but the bandwidth of the antenna element 24 is reduced.
A wound slot antenna element has to be fed at a location close to its end because the input impedance at its center is very high. The antenna element can be fed using a microstrip line printed on the other side of the substrate with a matching extension or a shorted via hole, as shown in FIGS. 1(a) and 1(b). A coaxial cable can also be used, where its outer conductor is connected to the ground area of the slot antenna and its inner conductor is shorted through the slot.
One of the current design challenges for making multifunction antennas includes providing a plurality of different antenna elements in a single compact structure. One particular application where multiple antennas are needed in a compact and low cost design is for a vehicle antenna that is used for all of GPS, satellite radio, advance mobile phone service (AMPS), PCS and group special mobile (GSM) systems. Combining so many antennas in a single structure provides various design challenges that have heretofore not been met in the art. One design challenge includes making some of the antennas, such as the GPS and the satellite radio antennas, circularly polarized with an upward looking beam to accommodate signals from satellites. Other antennas, such as the AMPS, PCS and GSM antennas, require omni-directional and vertically polarized radiation patterns to receive and transmit terrestrial signals. Thus, there is a need to provide all of the antennas on a common structure and still satisfy these needs.